1. Field of the Invention
The present invention relates to an Sn-based multilayer coated steel strip having improved electric-resistance weldability and improved corrosion-resistance for use in general-purpose food cans and beverage cans. The present invention also relates to a method for producing the Sn-based multilayer coated steel strip.
2. Description of the Related Arts
Recently, various manufacturing methods and design of the beverage cans and food cans have been increasingly developed. These container materials recently for such cans should be inexpensive and exhibit superior characteristics.
The electric resistance welding process, e.g., Soudronic welding process, is widely employed in can manufacturing, because of advantages such as a high material yield, and a bonding strength high enough that leakages due to a bonding failure are kept to an extremely low level, and that cans of various designs can be produced. Cans to be produced by the above welding process were heretofore made from Sn-plated steel sheet having an Sn plated amount of #10 (amount of plated Sn=1.12 g/m.sup.2) or more preferably #25 (amount of plated Sn=2.8 g/m.sup.2) or more. Nevertheless, a serious disadvantage of the Sn-plated steel sheet is that, due to increased tin prices, such a steel sheet has become expensive. Various attempts have been made to decrease the amount of plated Sn and thus attain a cost reduction, but the decrease in the amount of plated Sn cause a problem of degradation of the corrosion resistance and weldability. Recently, multilayer coated materials for container use have been developed, as disclosed in Japanese Unexamined patent publication Nos. 57-23,091, 57-200,592 and 57-110,685, as alternative materials to the Sn plated steel sheet. The production methods of these mutlilayer coated materials utilize various combinations of surface treatments of the steel sheet, i.e., Ni-plating, Sn-plating with a thin deposition amount, alloying-diffusion treatment of Ni-Sn (heating- and melting-treatment), and chromate treatment. The steel sheets so produced have a dual coating. In these steel sheets, due to the superimposing effects of the dual coating, the number of pinholes is reduced, and due to a dense formation of an Ni-Sn alloy layer, the ATC (alloy tin couple) value is lessened, and hence the corrosion resistance is enhanced. In these steel sheets, particularly when the underlying Ni plating layer is formed, the formation of an alloying layer of Fe and Sn (FeSn.sub.2 alloying layer), which occurs during the high temperature-heating step of the can manufacture involving welding or at the high temperature-sterlization step subsequent to filling of the can, is suppressed and the weldability and the appearance of the welded parts are improved.
When the known steel sheets for container use are considered in detail, it cannot be necessarily concluded that the requisite properties are ensured. Referring to FIG. 1, the dissolution speeds of Sn of the various Sn plated layers in the model corrosive liquid are shown.
The model corrosive liquid was a solution containing 1.5% of citric acid and 1.5% of sodium chloride. The dissolution test was carried out under the measurement condition of a temperature of 27.degree. C. and an N.sub.2 atmosphere.
The test samples had the following coating structures.
. . . Undercoating: (Fe-18% Ni-1.7% P) alloy plating (160 mg/m.sup.2).fwdarw.Sn plating (780 mg/m.sup.2).fwdarw.heating and melting treatment.fwdarw.chromate treatment (10 mg/m.sup.2). PA1 .circle. . . . Undercoating: (Fe-20% Ni) alloy plating (200 mg/m.sup.2).fwdarw.Sn plating (800 mg/m.sup.2).fwdarw.heating and melting treatment.fwdarw.chromate treatment (9 mg/m.sup.2) PA1 .DELTA. . . . Undercoating: Ni plating (25 mg/m.sup.2).fwdarw.Sn plating (800 mg/m.sup.2).fwdarw.chromate treatment (8 mg/m.sup.2) PA1 .quadrature. . . . Undercoating: (Fe-10% Ni) diffusion coating layer (Ni plating at 50 mg/m.sup.2 followed by diffusion treatment).fwdarw.heating and melting treatment.fwdarw.chromate treatment (8 mg/m.sup.2) PA1 x . . . Sn plating (850 mg/m.sup.2).fwdarw.heating and melting treatment.fwdarw.chromate treatment (9 mg/m.sup.2) PA1 . . . Undercoating: (Ni-16% P) alloy plating (60 mg/m.sup.2).fwdarw.Sn plating (850 mg/m.sup.2).fwdarw.chromate treatment PA1 A. No cracks formed on the surface coating layer due to rivetting or scoring, and even if cracks are formed, they do not reach the base steel. PA1 B. The lacquerability of the worked parts is not degraded.
As is understood from the , .DELTA., and .quadrature. curves, when the dual layer plated steel sheets comprising the Ni undercoating plating and the Sn plating are exposed to corrosive environments, the dissolution speed of Sn lessens at the initial corrosion stage, and hence, the initial corrosion resistance of these steel sheets is excellent. Nevertheless, when they are exposed to a corrosive environment over a long period of time, the Sn is consumed and the alloy layer may be exposed. Under such circumstances, no matter how dense the alloy layer, it is not free of pinholes, so that a local cell is formed and the corrosion is promoted. In the local cell, the Ni-Sn alloy layer is electric potentially extremely noble or cathodic relative to the steel base, with the result that the parts of the steel base exposed by the pinholes are preferentially dissolved. As a result, the corrosion resistance is degraded and, occasionally, piercing corrosion occurs. The piercing corrosion may occur because the exposed alloy layer or base steel is exposed due to flaws formed during the can manufacture. In this case, the base steel dissolves and the corrosion resistance is degraded.
Regarding the welding procedure, the speed of this procedure has been greatly increased, recent and accordingly, a higher weldability has become necessary. The amount of non-alloyed Sn (free Sn) is decisive when considering the weldability, and therefore, it is essential to suppress the reactions for the alloy formation occurring during the lacquer paint coating, thereby increasing the residual amount of free Sn. The underlying Ni plating of the current steel sheets used for containers, is effective to a certain degree in suppressing the alloy formation, but because of the high diffusion speed of Ni and Sn, it is diffucult to ensure a sufficient amount of free Sn is available for improving the weldability. Particularly, when the deposition amount of Sn is small, the excellent weldability needed to attain a high welding speed is not necessarily obtained by the underlying Ni plating.
Note, cans having easy-to-open ends (EOE) do not require cutting and can be easily opened anywhere. The EOE cans are used for all beverage cans and will probably be used for all food cans in the future. Al sheets provide a good end openable property and are widely used for the EOE materials. Surface treated steel sheets (tin plate) are used for foods cans to hold foods containing sodium chloride, for example, tomato juice, for which the Al cannot be used because of an insufficient corrosion resistance thereto. Recently, however, the materials of steel sheets and the designs for can ends have been improved, and tin plates having as good an openable property as the Al sheet have been produced for the ends of EOE cans. New materials, which will make it possible to reduce costs, are now required.
Not only a good weldability but also good lacquerability and, corrosion resistance after baking are needed for the materials used for welded cans. The materials used for the ends of EOE cans are subjected to score forming, i.e., the formation on the surface of an end, of a V-notch which facilitates can opening and allows the formation of a satisfactory opening for removing the content of the can therethrough. These materials are further subjected to bulging and the drawing of a tab, which acts as the starting point of tearing and staking, i.e., rivetting, for fixing the tab. Since the bulging, drawing, and rivetting are severe working, the steel sheet must have a good formability. In addition, the following properties are required for the surface coating layers.
Regarding ends other than the ends of EOE cans, and the can drums, the materials are subjected to severe working, such as winding-fastening by winding or bending, and thus the materials used must satisfy the same properties as described above.